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std::enable_nonlocking_formatter_optimization - cppreference.com

template< class T > constexpr bool enable_nonlocking_formatter_optimization = false;

(since C++23)

This template can be used by implementations to enable efficient implementations of std::print and std::println.

If std::enable_nonlocking_formatter_optimization<T> is true, printing an argument of type T can be performed in a more efficient way (see std::print for details). std::enable_nonlocking_formatter_optimization specializations can be true in the following cases:

T is one of the types where std::formatter<T, CharT> is a basic standard specialization or standard specialization for a library type (see below).
A program may specialize this template for any cv-unqualified program-defined type T. Such specializations must be usable in constant expressions and have type const bool.

[edit] Basic standard specializations

In the following list, CharT is either char or wchar_t, ArithmeticT is any cv-unqualified arithmetic type other than char, wchar_t, char8_t, char16_t, or char32_t:

Nonlocking flag for character formatters

template<> constexpr bool enable_nonlocking_formatter_optimization<CharT> = true;

(1)

Nonlocking flag for string formatters

template<> constexpr bool enable_nonlocking_formatter_optimization<CharT*> = true;

(2)

template<> constexpr bool enable_nonlocking_formatter_optimization<const CharT*> = true;

(3) template< std::size_t N > constexpr bool enable_nonlocking_formatter_optimization<CharT[N]> = true; (4) template< class Traits, class Alloc > constexpr bool enable_nonlocking_formatter_optimization <std::basic_string<CharT, Traits, Alloc>> = true; (5) template< class Traits > constexpr bool enable_nonlocking_formatter_optimization <std::basic_string_view<CharT, Traits>> = true; (6)

Nonlocking flag for arithmetic formatters

template<> constexpr bool enable_nonlocking_formatter_optimization<ArithmeticT> = true;

(7)

Nonlocking flag for pointer formatters

template<> constexpr bool enable_nonlocking_formatter_optimization<std::nullptr_t> = true; (8)

template<> constexpr bool enable_nonlocking_formatter_optimization<void*> = true;

(9)

template<> constexpr bool enable_nonlocking_formatter_optimization<const void*> = true;

(10) [edit] Standard specializations for library types

Specializations of enable_nonlocking_formatter_optimization for all specializations of the following standard templates are defined as true:

std::chrono::zoned_time when its template parameter type TimeZonePtr is const std::chrono::time_zone*

Specializations of enable_nonlocking_formatter_optimization for all specializations of the following standard templates are defined as conditionally true:

Specialization of enable_nonlocking_formatter_optimization for all formattable range types are always defined as false for which the range format kind is not std::range_format::disabled.

[edit] Notes Feature-test macro Value Std Feature __cpp_lib_print 202403L (C++26)
(DR23) Formatted output with stream locking 202406L (C++26)
(DR23) Enabling nonlocking formatter optimization for more formattable types [edit] See also defines formatting rules for a given type
(class template) [edit] prints to stdout or a file stream using formatted representation of the arguments
(function template) [edit] same as std::print except that each print is terminated by additional new line
(function template) [edit]

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